The present invention relates to an anti-wear additive for lubricating compositions and specifically to an extreme pressure, anti-wear additive.
Lubricating compositions reduce friction and reduce or prevent destructive contact between moving metal surfaces as long as a lubricating film is maintained between the moving surfaces. This particular type of lubrication is referred to as hydrodynamic lubrication.
Some anti-wear additives enhance the hydrodynamic lubrication of motor oils and the like. However, when the pressure and/or rubbing speeds between the moving metal surfaces increase, the lubricating film is forced out from between the moving metal surfaces. This results in metal-to-metal contact and wear. Lubrication under these extreme pressure conditions requires an additive that is adsorbed by or reacts with the metal to form an adherent protective film on the metal. This type of lubrication is needed under conditions called boundary lubrication, and additives enhancing this type of lubrication are known as "extreme pressure, anti-wear additives."
Many extreme pressure, anti-wear additives are known. The most commercially used additives are phosphorus-containing compounds, such as zinc dialkyldithiophosphates. While these phosphorus-containing compounds provide a high degree of boundary lubrication, there is a move away from this type of additive, especially for use in internal combustion engines. It is believed that oxidation products of phosphorus formed during combustion are carried by the exhaust gases into emission control catalysts where they reduce the life of the catalyst.
To replace phosphorus-containing compounds, various other types of extreme pressure, anti-wear additives have been used, such as the boron-containing compounds disclosed in U.S. Pat. Nos. 2,975,135, 3,347,793, 3,356,707, 3,509,054, and 4,115,286. These boron-containing compounds provide some degree of boundary lubrication, but they produce an ash upon combustion that can poison or plug emission control catalysts. Other additives of similar or superior properties to the dialkyldithiophosphates are still being sought, and particularly those which will not leave an ash residue to poison or otherwise interfere with automotive emission control catalysts.